Membrane vesicle fusion in eukaryotic cells is a highly regulated process coordinated through complexes of specific proteins. A variety of essential cellular functions, including protein transport and secretion, membrane protein distribution, and synaptic transmission, depend on regulated vesicle fusion. Many of the components which mediate the docking, priming, and fusion of membrane vesicles have been identified and characterized using mammalian synaptic nerve terminals as a model system. Homologues of these synaptic vesicle fusion proteins are located throughout the cell in the regions where intracellular membranes undergo fusion (Ferro-Novick, S., and Jahn, R. (1994) Nature 370:191-193).
In the nerve terminal, synaptic vesicles are tightly docked in specific locations near calcium channels on the presynaptic plasma membrane. During docking, synaptic vesicle proteins specifically complex with plasma membrane molecules, and this complex acts as a scaffold for the assembly of the fusion apparatus. The vesicle associated membrane protein binds to the plasma membrane syntaxin and SNAP-25 proteins. Once this trimeric core has assembled, it serves as a receptor for the cytosolic proteins .alpha.-SNAP and NSF. The ATPase activity of NSF then primes the synaptic vesicle-membrane complex for a calcium influx which will lead to vesicle fusion and extrusion of the vesicle contents.
When the nerve terminal is depolarized, Ca.sup.2+ enters via voltage-gated Ca.sup.2+ channels. High local concentrations of Ca.sup.2+ trigger the fusion of docked vesicles with the plasma membrane, which releases neurotransmitters into the synaptic cleft. A calcium-binding molecule, synaptotagmin, completes the process by interacting with syntaxin (Sudhof, T. C. (1995) Nature 375:645-653).
At least nine isoforms of synaptotagmin are expressed in brain and other organs. The members of this family are integral membrane proteins that span the vesicle membrane once and have a short amino-terminal intravesicular domain and a large cytoplasmic domain. The cytoplasmic domain contains two repeats homologous to the calcium-binding C2 domains found in Ca.sup.2+ -dependent isoforms of protein kinase C. Characterization of human and Drosophila synaptotagmins shows a selective conservation of C2 domains between the species. Nuclear magnetic resonance spectroscopy and site-directed mutagenesis showed that the interaction of synaptotagmin and syntaxin is mediated by the action of basic residues surrounding the Ca.sup.2+ -binding sites of the synaptotagmin C2A domain with the abundant acidic residues of the syntaxin molecule. Additionally, synaptotagmin specifically interacts with the cytoplasmic domains of neurexins, which are involved in many aspects of synapse organization. Inactivation of syntaxin and synaptotagmin with botulinum neurotoxin causes flaccid paralysis. (Sudhof, T. C. and Rizo, J. (1996) Neuron 17: 379-388; Ibaraki, K. et al. (1995) Biochem. Biophys. Res. Commun. 211:997-1005; and Perin, M. S. et al. (1991) J. Biol. Chem. 266:615-622).
The syntaxins display a broad tissue distribution, participate in vesicle docking with the presynaptic plasma membrane and in the regulated secretion of molecules, such as insulin, and regulate the potential targeting and fusion of carrier vesicles following export from the ER. Members of this family contain an N-terminal region exposed to the cytoplasm and C-terminal hydrophobic residues believed to function as a membrane anchor. The human syntaxin 1B gene has been mapped to 16p11.2 by fluorescence in situ hybridization. Chromosome rearrangements with breaks in 16p11 are observed in myxoid liposarcoma and in acute myeloid leukemia. Small cell lung cancer, a tumor that displays neuroendocrine properties, has been observed in about 60% of patients with Lambert-Eaton myasthenic syndrome, an autoimmune disease of neurotransmission that is characterized by muscle weakness. Analysis of Drosophila syntaxin mutants indicates that syntaxin is required for cell viability and may mediate membrane assembly events throughout development (Smimova, T. et al. (1996) Genomics 36: 551-553; Vincent et al. (1989) Trends Neurosci. 12:496-502; Hay, J. C. et al. (1997) Cell 89:149-158; and Schulze, K. L. and Bellen, H. J. (1996) Genetics 144:1713-1724).
The discovery of two new human membrane fusion proteins and the polynucleotides encoding them satisfies a need in the art by providing new compositions which are useful in the diagnosis, prevention and treatment of cancer and neuronal disorders.